As with many other reactions, the synthesis of colloidal semiconductor nanocrystals requires precise control of the reaction temperature. Usually, the synthesis starts at a relatively high temperature to produce a burst of nucleation events. After this, the reaction is quickly cooled to the growth temperature to allow the nuclei to grow without further nucleation. A clean separation between nucleation and growth is critical to reduce the size dispersion of the nanocrystal. However, such temperature control was difficult to implement in a microfluidic reactor. Semiconductor nanocrystal synthesis routinely requires temperatures as high as 300°C, which makes on-chip thermal management rather difficult.

Winterton et al. recently reported (J Nanopart Res 2008) a new microfluidic reactor design that could potentially maintain several different temperature zones within a single chip. The major improvement of the design is the integration of active cooling onto the chip. The microfluidic reactor is fabricated on Si wafer using standard lithography techniques. It consists of several coils of a micro-fabricated channel connected in series. Each coil of the channel is located on a coin-shaped island that connects to the rest of the wafer with a small tether. Outside the coin, the wafer is embedded in aluminum tubing that runs cooling water. This design thermally isolates the coiled channel, which carries out the reaction, from the rest of the chip. As a result, it is possible to keep the coiled channel at high temperature while keeping the rest of the wafer cool.

Using this reactor, nanocrystal nucleation and growth can be physically separated in two coiled channels that were heated to the nucleation temperature and growth temperature, respectively. By introducing additional precursors between coiled channels, fabrication of more sophisticated nanostructures, like core-shell quantum dots, is also possible. Unfortunately, the authors did not implement an on-chip heating device into their design. As a result, they have not yet carried out an actual nanocrystal synthesis using the reactor. By using external heating, however, the authors were able to show that the bulk wafer can be maintained below 80°C while keeping the coiled channel at temperatures as high as 450°C.